High breakdown voltage and low resistance characteristics are required for electrical power control devices (power semiconductor devices) used in switching power sources, inverters, and the like. There is a trade-off relationship between the voltage breakdown characteristics and the output characteristics of semiconductor devices that is determined by the properties of the materials of the device. Silicon has been used as a material for power semiconductors to-date. As a result of technical development over many years, a limit is being approached in the high breakdown voltage and low resistance characteristics of power semiconductor devices, and that limit is determined by the material properties of silicon. Therefore, devices using new materials instead of silicon, such as silicon carbide (SiC), nitride semiconductors, and the like, are being developed to overcome this limitation.
For example, it is known that by forming a heterojunction by stacking the nitride semiconductors gallium nitride (GaN) and aluminum gallium nitride (AlGaN), a 2-dimensional electron gas (2DEG) is formed at the interface. This 2DEG has a sheet carrier concentration of not less than 1×1013 cm−2, and a mobility of not less than 1,000 cm2/Vs. A Hetero-Structure Field Effect Transistor (HFET) using this as a channel has high breakdown voltage and low on-resistance, so it has been gaining attention as a device in which maximum characteristics of nitride semiconductors can be displayed.
However, a HFET that uses a 2DEG is a normally-on device in which current flows between the source and domain when no gate voltage is applied. A normally-off device is desirable from the points of view of safety and reducing electrical power consumption. In particular, for power semiconductor devices used for electrical power control, a normally-off device is essential for safety, so a nitride semiconductor device with normally-off properties is desirable.